Heat resistant alloy for carburization resistance

ABSTRACT

The catastrophic carburization of metal furnace tubes used in cracking and pyrolyzing hydrocarbons is minimized by forming a protective scale of manganese and chromium oxide on the surface of the metal. A nickel content of about 36 to 38% and a manganese content of 1.25 to 2.0% in a nickel-chromium-iron alloy promotes the formation of the oxide scale upon treatment of the alloy with steam.

United States Patent [191 Bagnoli et a1.

[ HEAT RESISTANT ALLOY FOR CARBURIZATION RESISTANCE [75] Inventors: Donald L. Bagnoli, Oak Ridge;

Anthony R. Ciuffreda, Colonia, both of NJ.

[73] Assignee: Exxon Research and Engineering Company, Linden, NJ.

[22] Filed: Aug. 13, 1973 [21] Appl. No.: 387,946

[52] US. Cl. 148/635, 75/128 A [51] Int. Cl. C23c 13/00 [58] Field of Search 148/635; 75/128 A [56] References Cited UNITED STATES PATENTS 2,543,710 2/1951 Schmidt et a] 148/635 X [4 1 Feb. 11, 1975 3,260,594 7/1966 Ornitz et a] 75/128 A Primary ExaminerCameron K. Weiffenbach Assistant Examiner-John D. Smith Attorney, Agent, or Firm-John .1. Dvorak [57] ABSTRACT 3 Claims, No Drawings HEAT RESISTANT ALLOY FOR CARBURIZATION RESISTANCE BACKGROUND OF THE INVENTION This invention relates to the pyrolysis of hydrocarbons. More particularly, it relates to a technique for minimizing carburization of furnace tubes exposed to hydrocarbons under pyrolysis and reforming conditions.

Processes for the controlled pyrolytic decomposition of hydrocarbons are well known. Typically, the hydro carbon is decomposed at high temperatures, for example in the range of l,700 to 2,000 F. under varying pressure conditions ranging from atmospheric up to about 1,200 psi. Such pyrolytic techniques are frequently used, for example, in producing ethylene. The hydrocarbons are cracked at elevated temperatures in the presence of predetermined amounts of steam. Pyrolytic techniques are also used for increasing the octane rating of gasoline.

It has been well recognized that the abovementioned cracking and reforming processes when conducted in the presence of certain metals and metal alloys often lead to excessive deposition of carbon. These carbon deposits not only tend to reduce the size of the furnace tubes with concomitant problems of plugging, but most importantly they also affect the strength of the metal furnace tubes resulting in structural failure of the metal tubes. In such instances such failure is frequently referred to as catastrophic carburization of the metal.

It is known for example that iron surfaces will have a catalytic effect upon the pyrolysis of hydrocarbons, promoting carbon formation. The carbon formed is absorbed or diffused into metal resulting in the structural failure of into the metal. Consequently, numerous techniques have been proposed for avoiding such catastrophic carburization of metal tubes by alloying the iron and/or by forming protective coatings of an oxide on the surface of the metal to be contacted with the hydrocarbon under the reforming or pyrolysis conditions. None of these techniques have provided a complete panacea and there is more than definite commercial interest in prevent carburization of furnace tubes exposed to hydrocarbons under pyrolysis and reforming conditions.

SUMMARY OF THE INVENTION According to the present invention, there is provided a method of heating hydrocarbon carbons in contact with metal surfaces under conditions which normally would cause difficulties due to carburization of the metal surfaces which comprises contacting the hydrocarbons of the metal surface containing adherent oxide layer of manganese and chromium, whereby heat is passed from the metal surface to the hydrocarbon without significant carburization of the metal.

Thus, in one aspect the present invention contemplates a method of rendering metal surfaces exposed to hydrocarbons under pyrolysis and reforming conditions resistant to carburization by providing on such metal surfaces in contact with the hydrocarbons and adherent layer of manganese and chromium oxide.

In yet another aspect of the present invention, an ad herent layer of manganese and chromium oxide is formed on a metal alloy surface by oxidizing a nickelchromium-iron alloy that has at least 36% nickel and from 1.25% to 2.0% manganese.

DETAILED DESCRIPTION OF THE INVENTION The present invention can be carried out for example by passing a hydrocarbon through a heating means having one or more tubes or conduits which are heated di- 'rectly or indirectly to transfer heat to the hydrocarbon. The metal surfaces to be used in accordance with this invention should have an adherent coating of manganese and chromium oxide.

In a particularly preferred embodiment of the present invention the metal tubes are formed from an alloy consisting essentially of chromium, nickel and iron and having at least l.25% manganese and at least 36% nickel. For example, the alloy may have from 36 to 38% nickel, from 23 to 27% chromium, and from 1.25% to 2% manganese. Indeed such an alloy when contacted with steam at elevated temperatures for a time sufficient to oxidize some of the manganese present in the alloy results in the formation of an adherent protective coating of manganese and chromium oxide which is resistant to carburization. For example, the metal surfaces are pretreated with steam at temperatures in the range of 500 F. to about 2,000 F., and preferably at about l,500 F. for from about 24 hours to about 96 hours, such as about 72 hours.

Equally important in providing an adequate protective coating of manganese and chromium oxide on the surface of the metal is the grain structure of the contact surface. It has been discovered that with furnace tubes, for example, the interior contact surface should be made up of equiaxed grain structure. Basically the grain structure is achieved by casting the alloy into a tube and controlling the thermal gradient during solidification. Any other technique known in the art for controlling grain structure can be employed.

The invention will be better understood by reference to the following examples and demonstrations.

EXAMPLE 1 TABLE I Alloy 1 Alloy 2 Carbon, 0.4-0.5 0.43 Manganese, 7r l.09 1.30 Silicon, L01 L10 Chromium, 7: 28.0 23.5 Nickel, 70 19.0 37.0 Molybdenum, 7c 0 L89 Iron, Balance Balance At the end of 30,000 hours on stream the furnace tubes were physically inspected. Those tubes of alloy l were, at least in some instances, carburized as much as and had as little as two-thirds of an effective wall thickness remaining. Those of alloy 2 were effectively protected against carburization. Magnetic readings of the tubes of both alloy 1 and alloy 2 showed that the protected regions were predominantly oxides of manganese and chromium whereas the unprotected areas were predominantly oxides of iron.

in a more detailed analysis performed with a microprobe analyzer, it was surprisingly discovered that a continuous layer of manganese oxide was lying outside the chromium oxide layer and close to the surface and the highly protected layers. Indeed, in alloy 2 the thickness of the manganese oxide scale was about times greater than the amount of manganese oxide scale in alloy 1.

Apparently, a nickel content, in a nickel-chromiumiron alloy, of greater than 36% and a manganese content of greater than 1.25% is necessary to provide a good protective scale of manganese and chromium oxide in the interior surface of the furnace tube.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications or variations may be resorted to without departing from the spirit and scope of the invention. Those skilled in the art will readily understand that such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

What is claimed is: l. The method of forming a protective scale of manganese and chromium oxide on the surface of alloy consisting essentially of manganese, chromium, iron and nickel comprising: 7

l. adjusting the nickel content of the alloy to the range of 36 to 38% and the manganese content of the alloy from 1.2 to 2.0%;

2. forming the alloy'into a metal surface; and

3. thereafter, treating the metal surface with steam at elevated temperatures for a time sufficient to oxidize manganese and chromium present in the alloy and form a protective scale of manganese and chromium oxide.

2. The method of claim 1 wherein the metal is treated with steam at temperatures in the range of 500 F. to 2,000 F. for at least about 24 hours.

3. The method of claim 1 wherein the metal is treated with steam at a temperature of about l,450 F. for

about 72 hours. 

1. THE METHOD OF FORMING A PROTECTIVE SCALE OF MANGANESE AND CHROMIUM OXIDE ON THE SURFACE OF ALLOY CONSISTING ESSENTIALLY OF MANGANESE, CHROMINUM, IRON AND NICKEL COMPRISING:
 1. ADJUSTING THE NICKEL CONTENT OF THE ALLOY TO THE RANGE OF 36 TO 38% AND THE MANGANESE CONTENT OF THE ALLOY FROM 1.2 TO 2.0%,
 2. FORMING THE ALLOY INTO A METAL SURFACE, AND
 2. forming the alloy into a metal surface; and
 2. The method of claim 1 wherein the metal is treated with steam at temperatures in the range of 500* F. to 2,000* F. for at least about 24 hours.
 3. The method of claim 1 wherein the metal is treated with steam at a temperature of about 1,450* F. for about 72 hours.
 3. thereafter, treating the metal surface with steam at elevated temperatures for a time sufficient to oxidize manganese and chromium present in the alloy and form a protective scale of manganese and chromium oxide.
 3. THERAFTER, TREATING THE METAL SURFACE WITH STEAM AT ELEVATED TEMPERATURES FOR A TIME SUFFICIENT TO OXIDIZE MANGANESE AND CHROMIUM PRESENT IN THE ALLOY AND FORM A PROTECTIVE SCALE OF MANAGANESE AND CHROMIUM OXIDE. 